This invention relates to a continuous process for the preparation of triorganosilyl end-blocked polydiorganosiloxane fluids using an acid activated clay as the catalyst. More specifically, the instant invention relates to the preparation of these siloxane fluids with reduced silanol content and reduced cleavage of organic groups.
Polydiorganosiloxane fluids with low silanol content exhibit such benefits as improved viscosity stability at elevated temperatures due to improved thermal stability. Chemical means for removing silanol end-blocking such as reaction with a silylating agent such as hexamethyldisilazane are known in the art. However, direct preparation of polydiorganosiloxane fluids with low silanol content from silanol-containing linear siloxane feeds is not so well known.
Triorganosilyl end-blocked polydiorganosiloxane fluids are well known materials in the art. The primary chemical route to producing these fluids is the equilibration (polymerization) of cyclopolydiorganosiloxanes, linear silanol end-blocked polydiorganosiloxanes, or mixtures thereof with an appropriate end-blocking agent such as a hexaorganodisiloxane or a short-chain length triorganosilyl end-blocked polydiorganosiloxane fluid in the presence of an acid or base catalyst. The product of the equilibration process is a mixture of the desired fluid and a lesser amount of cyclopolydiorganosiloxanes.
Japanese Patent Application No. 29868-1956, published May 30, 1959, discloses a process for the continuous production of organosiloxanes of high degree of polymerization, using acidic clay or clay activated with acid or alkali treatment as the equilibration catalyst. No mention is made of the acid or base content of the clay. Cyclopolydiorganosiloxanes and hexamethyldisiloxane are the only organosiloxane feeds mentioned. Temperatures disclosed are 75.degree. to 100.degree. C. It is disclosed that it is possible to apply pressure or vaccuum to facilitate the organosiloxane feed when the need arises.
Siciliano, U.S. Pat. No. 3,853,933, issued Dec. 12, 1974, gives a good summary of early preparation of these siloxane fluids. Early processes for equilibration of polydiorganosiloxane fluids were carried out with soluble acid or base catalysts. The primary difficulty in use of the soluble catalysts was the need to neutralize and to remove the catalyst residue after equilibration. Additionally, in the case of the use of a strong liquid acid such as sulfuric acid, cleavage of the organic substituents of the polydiorganosiloxane fluids attached to silicon atoms posed quality problems with the final fluid.
Workers in the field looked at acids that were chemically or physically bonded to a solid support to further improve the efficiency of the equilibration process for the preparation of triorganosilyl end-blocked polydiorganosiloxane fluids. Cation exchange resins were found to be somewhat suitable for batch processes in which the equilibrating siloxane materials were in prolonged contact with the resins. However, these cation exchange resins were less than suitable for continuous operations because they tended to require prolonged residence times. The cation exchange resins had the additional disadvantage of being quite expensive.
Another type of solid catalyst that was tried in a batch process was acid-treated carbon black. Early types of acid-treated carbon black were unsuitable for continuous equilibration processes since unduly long residence times were needed in a carbon black bed.
Siciliano discusses the use of acid-treated clays. Siciliano points out that acid-treated clays used by early workers were found unsuitable for various reasons, such as the requirement of long residence times in an acid-clay catalyst bed to effect equilibration. Even though many of these problems were solved, Siciliano points out, a process utilizing excessively high temperatures is necessary.
The invention disclosed by Siciliano is a continuous process for producing polydiorganosiloxane oils, the process comprises continuously passing a fluid siloxane mixture through a catalyst bed of acid-activated carbon black having a carbon to acid ratio of 10:1 to 40:1. The catalyst bed is maintained at 85.degree. to 150.degree. C. and the pressure in the bed is maintained at a pressure of 5 to 200 mm Hg. Residence time in the catalyst bed was anywhere from 10 minutes to 4 hours. Siciliano discloses that the equilibrated product from this carbon black bed contains from 100 parts per million to 1 weight percent water. No mention is made of silanol content of the equilibrated fluid.
Siciliano discloses a second, less preferred embodiment of the invention in which two catalyst beds in series, utilizing the acid-activated carbon black, described supra, are used to produce a product stream containing substantially no water. The first catalyst bed is as described, supra, The product from the first bed then passes to a second catalyst bed. The second catalyst bed is maintained at a temperature of 85.degree. to 150.degree. C. at atmospheric pressure. The residence time in the second catalyst bed is anywhere from 10 minutes to 2 hours. Siciliano discloses that the final fluid has a water content of less than 100 parts per million. The only mention made of silanol content of the final fluid is that it is negligible.
Siciliano et al., U.S. Pat. No. 3,853,934, issued Dec. 12, 1974, discloses a process for producing linear polydiorganosiloxane oils, using acid-activated hydroaluminum silicate. The process comprises continuously passing into a first column a siloxane feed mixture and a small amount of the acid-activated hydroaluminum silicate and a diatomaceous earth. No solids appear to be held in this first column. The first column is maintained at a temperature in a range from about 150.degree. to 200.degree. C. at an unspecified pressure. The residence time in the first column is anywhere from 0.5 to 5 hours. The product from the first column contains less than 1000 parts per million water. Siciliano et al., discloses that to operate the process the acid-activated hydroaluminum silicate must have an acid equivalent of 5 to 35 milligrams of KOH per gram of solid and must have a mesh size of 10 to 200 microns. Siciliano et al., go on to state that acidactivated clays other than hydroaluminum silicate and even hydroaluminum silicate that do not have the above specifications will not function in an effective manner in the continuous process of the present invention. The stream from the first column can pass to a surge tank where it is held 150.degree. to 200.degree. C. anywhere from 0.5 to 3 hours. The mixture from the surge tank then passes to a second column, packed with the same acid-activated hydroaluminum silicate, described supra. The siloxane mixture from the surge tank is fed to the second column at a pressure anywhere from 15 to 100 pounds per square inch gauge. The second column is maintained at a temperature of 150.degree. to 200.degree. C. The residence time of the siloxane mixture in the second column is anywhere from 10 minutes to 2 hours. Siciliano et al., discloses that the product from the second column has substantially no water. However, the silanol content is not quantified nor is the branching level of the final fluid mentioned in the invention of Siciliano et al.